When I take pictures of strangers it's always difficult for me to get over the feeling of prying. I generally try and take photos in a way that masks identities not only to be less obtrusive, but because it allows the viewer to imagine themselves in the photo you are talking.
Another reason I try and go unnoticed is to try and capture people before they see the camera. Once they do, any chance at something honest or candid is greatly reduced. Most people instantly change their demeanor when they are aware of being photographed.
My favorite photos of strangers are those that seem universal or evoke a common feeling or situation. This photo of a couple on the pier in Seattle made me think of being on a date and quietly enjoying a conversation. People generally project their own situation on a photo and I can see my wife and I enjoying the last few minutes of a sunset over a hot drink and brisk air in this photo, and it makes me want to go there.
People add so much to photos, but it takes courage to try and include them. I'm still developing mine.